Certain embodiments of the present invention relate to medical diagnostic systems, and in particular, to techniques and apparatus for adjusting the contrast of displayed diagnostic images acquired while using a collimator.
X-ray systems are well known for creating a series of internal images of a patient, such as cardiology, radiology and fluoroscopy systems. The patient is exposed to x-rays which are then detected after passing through the patient. The radiation is pulsed to produce a continuous sequence of images which are displayed real-time on a monitor. The x-rays are attenuated as they pass through the patient. The amount of attenuation experienced by the x-rays is represented in the image by the grayscale level of the pixels that are displayed. The contrast between grayscale levels is representative of the amount of attenuation.
Bones and different types of tissues attenuate the x-rays by different amounts, and thus are detected and displayed on an image monitor with different contrast levels. For example, bone will attenuate x-ray to a larger degree than muscle and may be displayed darker than surrounding anatomy. A region of anatomy containing only soft tissue may have a smaller range of contrast than a region of anatomy containing both soft tissue and bone. In addition, scattered radiation or using an increased kVp level to image a very large patient may also decrease the contrast range.
The level of radiation detected by the system is correlated to the contrast of the displayed image by a look-up table, or transfer function. In other words, the system uses the transfer function to assign a specific level of radiation to a specific grayscale level of the display. The system varies the range of the contrast for the displayed image associated with a particular range of grayscale levels by changing the shape (e.g., slope, offset, etc.) of the transfer function. The system may have multiple transfer functions, representing different mathematical models or shapes, from which one transfer function is selected to control the contrast of one or more display images. A particular transfer function is selected for different procedures or when imaging different anatomy.
The range of contrast used to display multiple images during a procedure may be set by the system and remain constant throughout the procedure. Therefore, a level of detected radiation, or a particular brightness level, is assigned a particular grayscale level for one or an entire series of scans. This is not advantageous, as during the same procedure, areas of interest within a patient which have various ranges of contrast may be scanned, and thus the contrast may appear to fluctuate. Some images may appear with a high level of contrast, containing areas that are very dark and areas that are very light, while other images may have low contrast and appear light or washed out. An operator may chose to adjust the contrast of the displayed image to correct for the change in contrast in the anatomy, but this is time consuming, error prone, and would need to be repeated as different tissues are examined.
To provide a more constant contrast throughout a procedure (fluoroscopic, cardiology, radiology or otherwise), automatic contrast compensation algorithms have been proposed. Automatic contrast compensation, or contrast management, is utilized to present a more pleasing image with better diagnostic utility. The images are examined for maximum and minimum brightness levels as they are acquired. The maximum and minimum brightness levels are then used to determine a new grayscale transfer function to enhance the contrast of the displayed image. Therefore, a radiologist may view images that contain bone and images containing only soft tissue during the same procedure without manually adjusting the contrast.
Unfortunately, automatic contrast compensation algorithms are also sensitive to data from regions where radiation has been blocked, such as when a collimator is used. A collimator may be used to block a portion or portions of an x-ray beam to minimize exposure to areas of the body which are not of diagnostic interest. When using a collimator, automatic contrast compensation, when enabled, identifies a minimum brightness level and adjusts the contrast such that the data detected from the collimator is assigned the lower end of the available grayscale, or the minimum brightness levels. Therefore, the grayscale range available to display the anatomic region of interest is decreased, and the displayed anatomic data reflects a sudden decrease in contrast.
In an effort to eliminate the effect of the collimator, it has been proposed to determine the maximum and minimum brightness levels from a small area of an image that will never be obstructed by the collimator regardless of where the collimator is actually located. Unfortunately, controlling brightness levels based on a smaller portion of the field of view may compromise contrast enhancement for larger fields of view, such as when no collimator or a less obstructive collimator is used. This method results in anatomic grayscale data being lost when imaging high contrast anatomy.
Thus, a need exists in the industry for a method and apparatus for adjusting the contrast of displayed diagnostic images when a collimator is used that addresses the problems noted above and previously experienced.
In accordance with at least one embodiment, an x-ray system is provided utilizing an x-ray source that irradiates a subject with x-rays. A collimator is located proximate to the x-ray source and blocks a portion of the x-rays. A detector receives the x-rays from the x-ray source and creates subject data indicative of a subject and collimator data indicative of a collimator. A position detector identifies a position of the collimator with respect to a field of view of the x-ray source. A gating module receives the subject and collimator data and passes at least a portion of the subject data. The gating module blocks at least a portion of the collimator data based on the position of the collimator. A display displays an x-ray image based on at least a portion of the subject data passed by the gating module.
In accordance with at least one embodiment, an x-ray system is provided utilizing an x-ray source to irradiate a subject with x-rays. A collimator blocks a portion of the x-rays and is located proximate the x-ray source. A detector receives x-rays from the x-ray source and creates subject data indicative of a subject and collimator data indicative of a collimator. A collimator calculation module identifies at least one of a position, an orientation, a shape, and a boundary of the collimator based on the position of the collimator. A gating module receives the subject and collimator data, and passes at least a portion of the subject data and blocks at least a portion of the collimator data based on the position of the collimator. A display displays an x-ray image based at least on the portion of the subject data passed by the gating module.
In accordance with at least one embodiment, a method for enhancing the contrast of an x-ray image is provide. A subject is exposed to an x-ray source which is partially blocked by a collimator. Subject data indicative of a subject and collimator data indicative of a collimator is detected. Position data indicative of a position of the collimator is identified. The subject and collimator data is gated to block the collimator data based on the position data. An x-ray image is displayed based on at least a portion of the subject data.